I am fitting a 2nd W.C. Toilet pan, up-stream, and on the same horizontal plane as the original. I need a 1:80 fall, I’m told. Do I raise the height of the new W.C. on a plinth of ply-wood? I’m using 110mm OSMA soil system in the U.K. Also, I am meeting the horizontal run with a new wash-basin outlet… I have 3 90Deg. bends.. what do you think? total run is 1.7M to vertical stack outlet.